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molarity and molality

Submitted by manijhis88 on Fri, 02/08/2008 - 14:47

A typical commercial-grade phosphoric acid is 75% H3PO4, by mass, and has d=1.57g/mL. What are the molarity and the molality of H3PO4 in this acid?

If it's 75% acid then it's 25% water (assuming an aqueous solution)

molality = moles solute / kg solvent
so assume a 100 gram sample of the acid solution, 75 grams must be acid.
Find the number of moles of H3PO4.
Convert 25 grams of water to kilograms.

Plug into the molality formula above.

For molarity you need to find the volume of the solution since
          molarity = moles of solute / volume of solution in liters

If we continue our assumption that we have a 100 gram sample of the solution we know the number of moles of H3PO4 contained in it from our calculation above.

We need the density to find the volume of the solution.
                       100 g solution  x   1 ml/1.57 grams = 63.7 mL or .0637 L

Plug the values into the molarity formula above and you have your answer.